Frequency modulation system



March 1 1949- s. FRANKEL FREQUENCY MODULATION SYSTEM 3 Sheets-Sheet l Filed Jan. 25, 1945 ASSE S.FRANKEL FREQUENCY MODULATION SYSTEM March l, 1949.

Filed Jan. 23, 1945 3 Sheets-Sheet 3 lll IN1/EN TOR. s/a/vfy ffm/wm BY ATToR/VEY Patented Mar. 1, 1949 UNITED STATES PATENT OFFICE FREQUENCY MODULATION SYSTEM Sidney Frankel, Forest Hills, N. Y., assignor to Federal Telephone and Radio CorporationfNew York, N. Y., a corporation of Delaware Application January 23, 1945, Serial No. 574,115

11 Claims. 1

This invention relates to electrical signalling systems and more particularly to frequency modulation.

'I'his invention is a continuation-in-part of my invention disclosed in my co-pending application Serial No. 507,107, filed October 21, 1943, for Frequency signalling generating system, assigned to Federal Telephone and Radio Corporation, now Patent No. 2,380,959, issued August '7, 1945.

`In the system disclosed in my aforesaid copending application telegraph or other similar signals are transmitted by changing the frequency of an oscillatory wave between two values after the wave has been produced. An important feature of the invention of my co-pending application is the employment of a stable oscillator the operation of which is unaffected by the sigoscillator will be unaffected, thereby producing a constant frequency, and upon application of positive and negative signal variations, the phase of the wave will be shifted in a continuous manner effectively producing frequency changes proportionately above and below this stable frequency.

Another feature of the invention is that it advantageously utilizes a reactance modulator tube to the grid of which the sawtooth voltage is applied. The frequencies produced by the very rapid voltage shift in the trailing `edge of the sawteeth are so high that they are supersonic and can be readily filtered out without affecting the main frequency of the `wave. Furthermore, these high frequencies are of such short duration that nals, the signals operating only to effect a shift I in phase after the wave leaves the oscillator.

It is an object of the present invention to provide means for frequency modulating a wave of predetermined frequency according to substantially the instantaneous values of a signal wave as distinguished from the "on and off characteristics of telegraph signals.

Another object of the invention is to provide modulator means for producing voltage undulations, the rates of voltage change of which are caused to vary in proportion to substantially the instantaneous values of a signal Wave.

One of the features of the invention is the production of a sawtooth Wave wherein the sawteeth are of a given short duration and the rate of voltage build-up, either positive or negative as the case may be, varies according to substantially the instantaneous value of a signal wave. The sawteeth thus produced are employed to modulate an oscillatory wave produced by a stable oscillator. The modulator circuit is arranged to change the phase of the wave according to the rate of change of the sawtooth voltage, the sawtooth being of the character having a build-up rate which is proportional to the signal values at corresponding instances. The degree of phase modulation of the wave is thus determined by the rate of voltage change represented by the leading edge of eachsawtooth. The ily-back interval of the trailing edge of the sawteeth is as rapid as is practical so that it has negligible effect upon the modulation of the wave. For a series of sawteeth where the steepness of the leading edges thereof increase from sawtooth to sawtooth, the wave modulated thereby will have corresponding increases in frequency. In the absence of a modulating signal, the wave of the they have a negligible overall effect on the signal component of the resultant wave.

The above and other objects and features of the invention will become better understood upon consideration of the following detailed description to be read lin connection with the accompanying drawings in which:

Fig. l is a schematic wiring diagram of a. transmitter according to the principles of my invention;

Fig. 2 is a graphical illustration useful in explaining the modulator operation of the embodiment of Fig. 1;

Fig. 3 is a schematic wiring diagram of a modified form of signal modulator; and

Fig. 4 is a graphical illustration useful in explaining the operation of the circuit of Fig. 3.

Referring to Figs. 1 and 2 of the drawing, oscillator I is preferably a crystal-controlled oscillator capable of generating a sinusoidal voltage at a supersonic frequency of say kc. This voltage is used to excite a generator 2 for producing sharp synchronizing pulses4 at the same fre quency. Since oscillators and pulse generators of this character are known to the art, a detailed description thereof is unnecessary.

The output of the synchronizing pulse generator is applied to the grids 3 and 4 of vacuum tubes 5 and 6 of two sawtooth generator circuits arranged in parallel. A condenser 'l is connected across the plate and cathode electrodes of tube 5 and a condenser 8 is likewise connected across the plate of cathode electrodes of tube 6. The signal energy is applied to the primary 9 of an input transformer I0, the secondary of which is connected between a midpoint II and ground I2 for application of energy through resistors I3 and I4 to the plate circuit I5 and the cathode circuit I6 of the tubes 5 and 6 respectively. In the present embodiment, the signal energy is the sole source of plate current. The cathode I1 of tube and the plate I8 of tube 6 are grounded so that the signai energy is caused to charge the condensers 1 and 8 in push-pull manner. In order to avoid improper charging of the condensers I and 8 during alternate polarity swings of the signal energy, the condensers 1 and 8 have connected thereacross diodes I9 and 20 respectively. These diodes act as a short circuit during the half period of the Wave in which a charging action is undesirable. The resistors 2Ikand 22 are provided in the output circuit 23, which is connected across resistors I3 and I4, to prevent inter-action between the two sides of the parallel generator arrangement.

In operation, synchronizing pulses from generator 2 such as illustrated at 24 in graph a of Fig 2 are applied to the grids 3 and 4 to control the tubes Sand 6 which normally are biased'to cut off as indicated at 25. The synchronizing pulses cause the tubes to conduct thereby drawing plate current stored in condensers 1 and 8. When a synchronizing pulse is removed from the grids 3 and 4, the tubes return to cutoff and the condensers 'I or. 8, as the case may be, recharge from energy source 9.

In graph b of Fig. 2, the signal energy is indicated as a sine wave 26. Graph c indicates the sawtooth undulations produced in the output circuit 23 in response to the pulses 24 and the signal wave 26.V For low positive values of signal energy increasing to higher values as indicated at 21, the sawtooth build-up varies proportionately as indicated at 28. For still higher values of signal energy, such as indicated by the wave portion 29, the build-up rate of the corresponding sawtooth increases proportionately as indicated at 30. It will be apparent from a comparison of the rising portions 21 and 29 of the signal energy and the leading edges of the sawteeth produced during these periods that the sawteeth increase in voltage at rates proportionately to the instantaneous values of the signal energy. Thus for increasing signal values the leading edges of the sawteeth saucer upwardly as shown by sawteeth 3I and 32, and for decreasing values of signal energy the rates of change of the leading edges decrease proportionately. I

For the negative swing of the signal-energy as indicated at 33, the sawteeth are produced by the other half of the sawtooth modulator and therefore are of negative polarity in contrast to the positive polarity of the sawtooth undulations 28, 38, 3| and 32. During the positive half of the signal wave the circuit branch including tube 5 operates to produce sawtooth undulations and during the negative swing of signal energy the branch including tube 6 operates to produce negative sawtooth undulations.

It will be clear from the graph c of Fig. 2 that the resulting sawtooth undulations include a signal amplitude component. This component is preferably filtered from the output by a high pass filter 35. I

The sawtooth voltage wave output of lter 35 is applied to the grid 36 of a reactance modulator tube 31 to .which is alsoy applied an oscillatory wave of predetermined frequency from a stabilized oscillator such as the crystal oscillator 38. The modulator circuit of tube 31 operates as a phase shifter for the oscillatory wave in ac- 4 output of amplifier tube 39, whose output circuit 40 is connected to the grid 36 by a suitable feedback circuit 4I arranged to provide the necessary ninety degree phase shift. The feedback circuit disclosed is of a type in which any tendency toward deviation from the required phase shift can be corrected by proper design or adjustment of the resistor 42. The circuit may utilize the gridcathode internal capacity of tube 31, indicated at 43, or a corresponding external capacitor if the internal capacity is not of suiiiciently high value.

The output of the oscillator 38 is applied to grid 44 of tube 39. The oscillator may be of conventional type, including a tube 49 connected to an oscillating circuit 46 including a piezo-electric frequency-controlling crystal 41'. The output from tube 39 is conducted to terminal 48 which may be connected to a power amplier 4S and antenna 50.

Should a continuous uniform voltage shift be applied to the grid 36 of tube 31 such as represented by a linearlyincreasing voltage ,of the leading edge of an ideal sawtooth, the tube circuit will produce a corresponding uniform and continuous phase shift in the amplified waves from oscillator 38 in the output of tube 39, so that an output of a different frequency is supplied to the power amplifier 49. The very high frequency shifts similarly produced momentarily during the abrupt voltage drop at the end of each sawtooth undilsation are ltered .out in the power amplier It follows that any change in rate of voltage increase of the leading edge of the sawtooth such cordance with the rate of voltage change applied as produced by the sawtooth modulator of Fig. 1, and indicated in graph c of Fig. 2, a corresponding phase shift is produced upon the Wave from oscillator 38. This phase shift results yin frequency modulation of the oscillatory wave as indicated by the wave 5i of graph d, Fig. 2. The frequency variation of wave 5I corresponds substantially directly to the rate of change of the leading edges of the sawteeth of graph c, the positive sawtooth undulations producing a phase shift in one direction and the negative sawtooth undulations producing a phase shift in the opposite direction with respectto the stable frequency Wave from oscillator 38.

For a further discussion of the operation of the reactance modulator and phase modulated amplifying circuits, reference may be had to my aforesaid co-pending application Serial No. 507,107 now Patent Number 2,380,959.

In Fig. 3, I show a simplied embodiment of the sawtooth modulator whereby the sawtooth undulations are of the same polarity regardless of the positive and negative swings of the audio input. The circuit is substantially the same as described in connection with the upper circuit of the sawtooth modulator of Fig. l with the exception that the diode I9 is omitted and a positive bias has been inserted as indicated at 52 in series with the secondary of the audio input transformer I0. The rest of the elements of the circuit .correspond to elements of they previously described modulator circuit as indicated by like reference characters.

In Fig. 4 graph e represents for Fig. 3 the synchronizing pulses 24 received from generator 2 while graph f shows the audio wave 26 applied to the circuit through transformer I0. The relationshipl of the bias provided at 52 is represented by the voltage difference of levels 53 and 54, the latter corresponding to the maximum negative swing of the audiowave 26. The wave 2l thus operates as a positive voltage throughlout and produces positive polarity sawtooth undulations such as indicated at 55 in graph g. When the voltage undulations produced by the modulator of Fig. 3 are applied to the reactance modulator tube 31 of Fig. 1, the wave produces a phase shift upon the oscillatory from oscillator 3B in one polarity direction only. While the sawtooth modulator of Fig. 3 is a much simpler circuit, lt has a disadvantage over sawtooth modulator of Fig. 1 in that for zero audio input the modulator of Fig. 3 continues to produce sawtoothvoltage undulations, which in turn phase modulate the stable frequency wave produced at 38; also, the phase shift in one direction only, as indicated by the unidirectional sawteeth of graph g.

While I have shown and described the principles of my invention in connection with specific embodiments, it should be understood that the I pulses for terminating the integrating process,

specific embodiments are intended for illustration purposes only and not as a limitation upon the scope of the invention as set forth in the objects.

I claim:

1. In a modulator, generator means to produce voltage undulations of generally sawtooth pattern, means for producing timing pulses to control the operation of said generator means, means to further control said generator means to vary the rate of voltage change of the undulations produced thereby according to substantially the instantaneous value of a signal wave, means to produce a wave of a predetermined frequency,

and means to vary the instantaneous frequency of said wave according to the rates of voltage change of said undulations.

2. In a modulator, generator means to produce voltage undulations of generally sawtooth pattern, means for producing synchronizing pulses, means'to control said generator means according to the timing of said synchronizing pulses, means to apply signal energy as the driving voltage for said generator means, whereby the rates of voltage change of a given edge of the undulations produced are proportional to substantially the instantaneous values of said signal energy, means to produce a wave of a predetermined frequency, and means to vary the instantaneous frequency of said wave in accordance with the rates ofv voltage change of said undulations.

3. In a modulator, generator means to produce voltage undulations of generally sawtooth pattern, means for producing timing pulses to control the operation of said generator means, means to further control said generator means to vary the rates of change of the voltage undulations produced thereby according to substantially the instantaneous value of a signal wave, means to remove any signal amplitude component present in said undulations, means to produce a wave of predetermined frequency, and means to vary the instantaneous frequency of said wave according to the rates of voltage change of said undulations after removal of said signal amplitude component.

4. A modulation signal source comprising an integrating circuit, a source of sinusoidal waves, means for applying said Waves to said circuit to be integrated over discreet intervals of time, a source of timing pulses having a separation corresponding to said intervals of time, means for applying said pulses to said circuit, said circuit responsive to the arrival of each of said n' un and responsive to the passage of said pulses for commencing the integrating process.

5. An arrangement according to claini4 4, further comprising a source of carrier frequency waves and means for angle modulating said carrier frequency waves with the integrated waves from said integrating circuit.

6. An arrangement according to claim 4, wherein said integrating circuit comprises valve means, a condenser and a resistor, means to control the operation of said valve means to discharge said condenser according to the occurrence of said pulses, means to apply said waves through said resistor to charge said condenser, and an output circuit connected across said resistor.

7. In a modulator, means including valve means, a condenser and a resistor to produce voltage undulations of generally sawtooth pattern, a source of timing pulses, means to control the operation of said valve means to discharge said condenser according to the occurrence of said pulses, means to apply a signal voltage through said resistor to charge said condenser, and means to apply a given bias voltage in series with said signal voltage, said bias voltage being at least equal to the maximum negative voltage variation of said signal.

8. In a modulator, means including valve means, a condenser and a resistor to produce voltage undulations of generally sawtooth pattern, a source of timing pulses, means to control the operation of said valve means to discharge said condenser according to the occurrence of said pulses, means to apply a signal voltage through said resistor to charge said condenser, means to apply a given bias voltage in series with said signal voltage, said bias voltage being at least equal to the maximum negative voltage variations of said signal, a high pass filter means to apply voltage undulations produced across said resistor to said lter for removal of any signal amplitude component present in said undulations, means to produce an oscillatory Wave, and means to vary the frequency of said wave in accordance with said voltage undulations.

9. In a modulator, first and second sawtooth generators each including valve means, a condenser and a resistor for producing voltage undulations of generally sawtooth pattern, a source of timing pulses, means to control the operations of the valve means of both said generators according to the occurrence of said pulses to discharge the condensers associated with said valve means. means to apply a signal voltage in parallel across the resistors of said generators to charge the condensers thereof, and output circuit means connected across the resistors of said generators.

10. In a modulator, a first sawtooth generator including valve means, a diode, a condenser and a resistor for producing voltage undulations of generally sawtooth pattern, a second sawtooth generator including valve means, a diode, a condenser and a resistor to produce voltage undulations of generally sawtooth pattern, a source of timing pulses, means to control the operations` of the valve means of both generators according to the occurrence of said pulses to discharge the condensers associated with said valve means, and means to apply a signal voltage in parallel across the resistor of said generator to charge the condensers thereof, the diodes of said generator being connected across the condensers of the generators so as to short circuit the condenser of the first generator for one polarity/f'V swing of the signal voltage andto short the condenser of the second generator when the signal voltage swings to the opposite polarity.

11. In a modulator, a rst sawtooth generator including valve means, a diode, 'a condenser and a resistor for producing voltage undulations of generally sawtooth pattern, a second sawtooth generator including valve means, a diode, a condenser and a resistor to produce voltage undulationsof generally sawtooth pattern, a source of timing pulses, means to control the operations of the valve means of both generators according to the occurrence of said pulses to discharge the condensers associated with said valve means, means to apply a signal voltage in parallel across the resistor of said generator to charge the condensers thereof, the diodes of said generator being connected across the condensers of the generators so as to short circuit the condenser of the first generator for one polarity swing of the signal voltage and to short the condenser of the second generator when the signal Voltage swings to the opposite polarity, a high pass lter,

means to apply voltage undulations produced across the resistors of the generators to said lter for removal of any signal amplitude component present in said undulations, means to produce an oscillatory wave, and means to vary the phase of said wave in accordance with said voltage undulations.

SIDNEY FRANKEL.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 2,284,401 Manley et a1. May 26. 1942 

